1. Field of the Invention
This invention relates to a butterfly valve for controlling high-temperature fluid, and more particularly to a butterfly valve for controlling high-temperature fluid which includes a ceramic valve.
2. Description of the Prior Art
In steel making, hot compressed air having a temperature as high as, for example, 1200.degree. C. and a pressure as high as, for example, 3.5 kg/cm.sup.2 (gage pressure) is fed to a blast furnace through number of tuyeres thereof at a high velocity of, for example, 200-250 m/sec. Also, the tuyeres provided at the blast furnace amounts to approximately thirty or more. Further, it is desired to control the flow of the compressed air of high temperature, high pressure and high velocity separately in each tuyere.
For this purpose, a valve structure using a metal valve was proposed and attempted. However, this valve was not put to practical use due to insufficient heat resistance. In order to eliminate such a problem, it was attempted to cool the metal valve with water. Unfortunately, however, this was not practical due to large heat loss. In view of the foregoing, it was proposed to have a valve made of a heat-resistant ceramic material for application to a butterfly valve which is of relatively simple structure.
A butterfly valve generally includes a valve which comprises a valve disc and a revolving valve stem, and the valve disc is rotatably supported within a fluid passage defined in a casing so that the fluid flowing through the fluid passage may be controlled by the rotation of the valve disc using the valve stem. The butterfly valve constructed as described above is assembled in such a manner that the valve disc is first arranged in the fluid passage of the casing, the valve stem is then inserted through a bore provided perpendicular to the flow direction of the fluid, and finally the valve disc and valve stem are connected together by means of bolts in the fluid passage of the casing.
However, when the valve disc and valve stem which are separately formed of a ceramic material are connected together in the fluid passage by bolts, it is required to provide both the ceramic valve disc and ceramic valve stem with boltholes. However, it is highly troublesome to provide the ceramic members with such holes, which cause the concentration of stress at portions of the ceramic members at which the holes are formed, and the ceramic members are liable to be damaged and/or broken. Also, ceramic bolts are generally difficult to obtain, and thus metallic bolts must be used instead of the ceramic bolts. This deteriorates excellent heat-resistant properties of the ceramic members. In a possible further alternative, it has been proposed to weld the ceramic valve disc and ceramic valve stem together in the fluid passage. However, as a matter of fact, the welding is impossible, because it requires a heat treatment at a high temperature.
In any case, it was conventionally highly difficult to have the ceramic valve disc and ceramic valve stem jointed in a narrow fluid passage so as to provide the valve with sufficient high-temperature strength. Thus, a butterfly valve using the ceramic valve has not yet been put into practical use.
In view of the foregoing, the inventors have proposed a butterfly valve which is constructed of a valve disc and a valve stem integrally formed of a ceramic material, and the valve thus formed is housed in a heat-resistant casing. This butterfly valve is satisfactory in heatresistance and high-temperature strength even when it is used to control the flow of fluid such as air of high temperature, high pressure and high velocity.
However, in the butterfly valve comprising the valve disc and valve stem which are integrally formed of ceramics, it is required to project the valve stem from the casing to connect the valve stem to a drive shaft when the valve is assembled in the casing. The casing is provided on the inside thereof with a heat insulating member formed of a refractory material, and the valve stem is supportedly inserted through a bore formed at the heat insulating member. Thus, the upright position of the valve stem in the casing is variable depending upon the working and assembling precision of the valve disc and heat insulating member. However, it is subtantially impossible to fabricate and assemble the valve disc formed of ceramics and the heat insulating member formed of refractory with substantially the same precision as those formed of metal, which results in the position at which the valve shaft is supported is considerably varied depending upon the working and assembling precision. This renders the precise axial alignment between the valve stem projecting from the casing and the driving shaft highly difficult. As a result, the valve stem is often rubbed against the wall of the bore of the heat insulating member and is worn and/or damaged due to the axial deviation occurring when the valve disc is rotated by the driving shaft.
The heat-resistant casing used for the conventional butterfly valve generally comprises a metal case and a heat insulating layer which is formed of refractory materials and lined on the inner surface of the metal case. As the refractory for heat insulating layer, castable refractory is generally used, which is formed into a single layer of a cylindrical shape and is applied to the inner surface of the metal case.
However, the conventional casing constructed as above is disadvantageous in that, when the interior of the casing is heated to an elevated temperature by hightemperature gas flowing therethrough, the inner and outer peripheries of the heat insulating layer are subjected to compression stress and tension stress due to heat expansion. As a result, a crack and the like is created at the heat insulating layer. In order to eliminate such a problem, it is proposed to use a heat insulating layer which is formed by combining a plurality of refractory pieces arcuate in section into a cylindrical shape. However, the heat insulating layer of this type permits heat to be transmitted to the metal case through joints between each two adjacent refractory pieces, from the fluid passage to the inner surface of the metal case, resulting in damage of the metal case and/or leakage of high-temperature gas from the casing.
Further, although the heat insulating layer applied to the metal case of the butterfly valve for controlling high-temperature fluid is required to have enough strength and heat-resistance sufficient to support the valve rigidity in a fluid stream of high temperature and high pressure, there is a general tendency that refractory having high strength is inferior in heat-resistance, whereas refractory of enough heat-resistance is low in strength.